Tolerance ring assembly

ABSTRACT

A tolerance ring having a first axial end, a second axial end, and a central axis, the tolerance ring including an annular band; a plurality of protrusions, all of the plurality of protrusions extending from the annular band; and a guide portion extending from the annular band towards the first axial end of the tolerance ring, where the guide portion includes a guide surface that is curved in an axial section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 16/235,199, entitled “METHOD OF USING A TOLERANCE RING,” by Niki S. WOODHEAD et al., filed Dec. 28, 2018, which is a divisional of and claims priority to U.S. patent application Ser. No. 14/636,550, entitled “METHOD OF USING A TOLERANCE RING,” by Niki S. WOODHEAD et al., filed Mar. 3, 2015, now U.S. Pat. No. 10,203,004, which application is a continuation of and claims priority to U.S. patent application Ser. No. 10/552,875, entitled “TOLERANCE RING ASSEMBLY,” by Niki S. WOODHEAD et al., filed Oct. 13, 2005, now U.S. Pat. No. 9,206,853, which was a U.S. national phase application under 35 U.S.C. § 371 of International Patent Application No. PCT/GB2004/001681, entitled “TOLERANCE RING ASSEMBLY,” by Niki S. WOODHEAD et al., filed Apr. 16, 2004, and claims priority to Great Britain Patent Application No. 0308957.0, entitled “TOLERANCE RING ASSEMBLY,” by Niki S. WOODHEAD et al., filed Apr. 17, 2003, of which these applications are assigned to the current assignee hereof and incorporated herein by reference in their entireties. The International Application was published in English on Nov. 4, 2004 as WO 2004/094852 A1 under PCT Article 21(2).

BACKGROUND OF THE INVENTION Field of the Disclosure

This invention relates to tolerance ring assemblies, wherein a tolerance ring provides an interference fit between parts of an assembly, where a first part has a cylindrical portion located in a cylindrical bore of a second part. The invention particularly relates to assemblies having a tolerance ring that provides an interference fit between a cylindrical component such as a shaft or a bearing and a housing for the shaft.

Description of the Related Art

Improved engineering techniques have resulted in the need for greater accuracy of machine parts, raising manufacturing costs. Very close tolerances are required where press fits, splines, pins or keyways are employed to transmit torque in applications such as pulleys, flywheels or driveshafts.

Tolerance rings may be used to provide an interference fit between parts required to transmit torque. Tolerance rings provide a low cost means of providing an interference fit between parts that may not be machined to exact dimensions. Tolerance rings have a number of other potential advantages, such as compensating for different linear coefficients of expansion between the parts, allowing rapid apparatus assembly, and durability.

A tolerance ring generally comprises a strip of resilient material, for example a metal such as spring steel, the ends of which are brought together to form a ring. A band of protrusions extend radially outwards from the ring, or radially inwards towards the centre of the ring. Usually, the protrusions are formations, possibly regular formations, such as corrugations, ridges or waves.

When the ring is located in the annular space between, for example, a shaft and a bore in a housing in which the shaft is located, the protrusions are compressed. Each protrusion acts as a spring and exerts a radial force against the shaft and the surface of the bore, providing an interference fit between the shaft and the housing. Rotation of the housing or the shaft will produce similar rotation in the other of the shaft or the housing, as torque is transmitted by the tolerance ring. Typically, the band of protrusions is axially flanked by annular regions of the ring that have no formations (known in the art as “unformed regions” of the tolerance ring).

Although tolerance rings usually comprise a strip of resilient material that is curved to allow the easy formation of a ring by overlapping the ends of the strip, a tolerance ring may also be manufactured as an annular band. The term “tolerance ring” as used hereafter includes both types of tolerance ring. The term “shaft” as used hereafter includes any assembly component with a cylindrical portion, such as a shaft or a bearing.

Problems can occur during assembly of parts that use tolerance rings. As the tolerance ring requires a tight fit in the apparatus, there may be abrasion between the ring and various parts of the apparatus, which removes small fragments from the surface of the affected parts. These fragments are known in the art as particles. In particular, parts of the protrusions distal to the band of the ring may generate particles when sliding relative to part(s) of the apparatus. In certain apparatus, such as a computer hard disk drive where cleanliness is essential, production of particles is extremely undesirable, as the particles can adversely affect the function of the apparatus.

If the material of the housing (e.g. aluminum) is softer than the material of the shaft (e.g. steel), it may be desirable to have the protrusions slide relative to the shaft during assembly to minimise particle production. Normally, for this type of arrangement, the tolerance ring's protrusions point inwards.

For apparatus that includes a bearing, however, it may also be desirable that the bearing has the tolerance ring fitted thereto, and the protrusions engage the surface of the bearing in the housing i.e. the protrusions point outwards. The arrangement may be useful where the tolerance ring distributes torque from the housing to the bearing, as load is distributed evenly over a large contact area of the shaft provided by the tolerance ring. Where the surface area is not maximized, for example when the protrusions engage the bearing under load, there may be problems with an effect known as “torque ripple” where the torque in the bearing is not generated at a continuous level. It is preferable that load is distributed over as large an area as possible, for example over the base of the protrusions of the tolerance ring.

It may be advantageous to provide in the bore a tolerance ring with outward protrusions and fit the shaft into the ring. In this arrangement, although there may be reduced particle production as the protrusions do not slide relative to an apparatus component, particles may still be produced, as the end of the shaft may rub against the rim of the tolerance ring during assembly owing to the tight fit between the tolerance ring and the shaft.

Alternatively, it may be advantageous to fit a tolerance ring with inward protrusions to the shaft, and insert the shaft and tolerance ring into the bore. Although the inward protrusions do not rub against the surface of the bore, particles may be produced when the rim of the tolerance ring rubs against the edge of the bore in the housing, owing to the tight fit between the ring and the housing.

The need to reduce or eliminate particle production can therefore impose restrictions on the type of tolerance ring used. The tolerance ring that minimizes particle production may not be the best choice for operation of the apparatus.

SUMMARY

Therefore, at its most general, the present invention proposes that the tolerance ring has a guide portion defining a surface inclined relative to the axis of the tolerance ring which can act as a guide when the tolerance ring is inserted in a bore or when a shaft is mounted in the tolerance ring. In the former case, the surface will be inclined outwardly whereas in the latter case it will be inclined inwardly. In either situation, the guide portion assists the assembly of the various components, but reduces or eliminates particle production.

Thus, according to a first aspect of the invention, there may be provided a tolerance ring comprising a band of resilient material having corrugated protrusions extending either radially outwards from the band or radially inwards towards the axis of the band; and a guide portion contiguous with, and extending axially from the band, wherein guide portion comprises at least one guide surface inclined relative to the axis of the band such that the guide free end of the guide portion is wider than the opening of the band when the corrugated protrusions extend radially outwards, and is narrower than the opening of the band when the corrugated protrusions extend radially inwards.

The angle of inclination of the at least one guide surface relative to the axis may be constant along the length of the guide surface, so that the guide surface defines the major part of a frusto-cone. Alternatively, the angle of inclination may change so that the guide surface is curved in an axial section.

An advantage of the present invention is correct assembly of a ring in a bore despite some axial misalignment of the ring and the bore.

The tolerance ring may be made of any suitable resilient material, preferably a metal, most preferably spring steel.

The guide portion may extend from the whole circumference of the band of the ring. Alternatively, the guide portion may extend from at least one segment of the circumference of the tolerance ring. The guide surfaces of the guide portion may be flared relative to the inside surface of the band of the tolerance ring. In one embodiment, the guide surface flares from the band to define an entrance at one end of the tolerance ring, wherein the perimeter or circumference of the entrance is greater than the circumference of the band of the tolerance ring. The guide portion provides a funnel extending from the band of the ring, with the guide surfaces angled relative to the axis of the band.

Conveniently, the guide surfaces of the guide portion are sufficiently free of obstruction so as to guide a shaft inserted axially into the guide portion into the band. If there is axial misalignment of the shaft and the axis of the tolerance ring, the guide surfaces of the guide portion serve to align axially the shaft with the band as the shaft is inserted into the band. The guide portion may include walls that are substantially parallel to the axis of the ring, for example near to the entrance.

Preferably, the entrance of the guide portion is circular or oval shaped. The entrance may be concentric with the band. The entrance may be rectangular or square in shape.

Preferably, the plane of the entrance is substantially perpendicular to the axis of the tolerance ring. Alternatively, the plane of the opening may be slanted relative to the axis of the ring, and a segment of the entrance is axially closer to the band than the rest of the entrance.

Alternatively, the guide surfaces of the guide portion taper towards the axis of the ring to narrow or close the opening of the band. The guide surfaces are provided by the outside surfaces of the guide portion. Ideally, the guide surfaces are sufficiently free of obstruction so as to allow the tolerance ring to be guided into a bore of a housing such that the bore of the housing is concentric with the band of the tolerance ring.

Ideally, the guide surface of any guide portion of the present invention is sufficiently smooth to prevent the production of particles when the guide surface rubs or slides against a shaft and/or the entrance to a bore in a housing. A possible advantage of the present invention is therefore a reduction in the particles normally generated when the tolerance ring is inserted inside a bore and/or fitted onto other apparatus, such as a shaft.

The tolerance ring of the present invention is particularly suited to apparatus where particle production is undesirable, for example the hard disk drive of a computer. Additionally, or alternatively, the present invention may be particularly useful where it is advantageous to distribute tolerance ring loading over as wide an area as possible on a component that has to be inserted into or over the tolerance ring.

Such an apparatus may comprise a housing, which includes a bore, in which bore a tolerance ring of the first aspect of the present invention is fitted such that the band of the tolerance ring is concentric with the bore and the guide portion extends and flares towards the mouth of the bore. The protrusions of the ring preferably extend radially outwards to engage the surface of the bore. A shaft can be inserted into the ring via the entrance provided by the guide portion, to be gripped by the ring. The internal diameter of the band is suitable to grip the shaft. Ideally, the internal diameter of the band is substantially the same as the diameter of the shaft.

The perimeter or circumference of the opening may be of sufficient dimension(s) to allow the opening to be accommodated inside the bore, so that no part of the guide portion protrudes out of the housing.

Alternatively part of, or the entire, guide portion may reside outside of the housing when the ring is in use, owing to the dimension(s) of the opening and/or the axial length of the guide portion. Alternatively, the guide portion may reside inside the housing such that the entrance of the guide portion lies flush with the outside of the housing. Preferably, the entrance of the guide portion is the same dimension and shape as the mouth of the bore.

The housing may be an actuator arm of a hard disk drive. The actuator arm may pivot relative to a bearing or bearing assembly.

The apparatus may comprise a shaft, onto which a tolerance ring of the present invention is fitted, wherein the protrusions of the ring preferably extend radially inwards. The tolerance ring may be fitted to the periphery of the shaft such that the protrusions engage and grip the shaft. The guide portion extends axially, away from the end of the shaft, and tapers to narrow or close the opening of the band. The guide portion preferably narrows the opening of the band but preferably does not taper to a point. Alternatively, the guide portion may taper to a point so that the tolerance ring acts a cap over the end of the shaft. Regardless of the exact shape of the guide portion, the guide surfaces of the guide portion allow the ring (and shaft) to be inserted into the bore of a housing such that the band of the tolerance ring sits concentrically in the bore. The external diameter of the band is sufficient to create a snug fit between the band and the bore. Ideally the external diameter of the band is substantially the same as the diameter of the bore.

Alternatively the apparatus may comprise a housing having a bore, a shaft and a tolerance ring of the first aspect of the invention, wherein at least part of the shaft is located in the bore and the band of the tolerance ring is concentrically located between the part of the shaft in the bore and the surface of the bore.

Preferably the apparatus is a hard disk drive and the shaft is a bearing or bearing assembly and the housing is an actuator arm. Preferably, the actuator arm pivots around the bearing or bearing assembly. Preferably the bearing or bearing assembly remains stationary.

Alternatively, the apparatus may be a torque limiting apparatus such as EPAS or a steering column lock, where slip occurs between the ring and another surface, an electric generator, a centrifugal pump, an air blower, a fan, a crane, woodworking or spinning machinery, a machine tool, a shaker or crusher.

The apparatus may be assembled by fitting a tolerance ring of the first aspect of the invention to a shaft, so that the inwardly extending protrusions of the ring grip the shaft and the guide portion tapers away from the shaft, the guide portion narrowing or closing the opening of the band; and inserting the shaft and ring into the bore of the housing such that the guide portion axially leads into the bore, such that the shaft/tolerance ring sits concentrically inside the bore and the band is sufficiently dimensioned to allow a tight fit between the band and the surface of the bore.

The guide surfaces of the guide portion may correct any axial misalignment between the shaft/ring and the bore.

Alternatively, the apparatus may be assembled by fitting a tolerance ring of the first or second aspect of the invention into a bore of a housing such that the outwardly extending protrusions of the tolerance ring contact the surface of the housing, wherein the band of the tolerance ring is suitably dimensioned to receive and grip a shaft, and the guide portion extends towards the entrance of the bore, the guide portion widening the opening of the band; and inserting a shaft into the guide portion such that the shaft slides into the band/bore to implement an interference fit between the ring and the shaft.

The guide surface(s) of the guide portion may correct any axial misalignment between the shaft and the tolerance ring/bore.

There may be a chamfer or flare at the mouth of the bore relative to the sides of the main section of the bore. This chamfer may act as a further guide, particularly in embodiments which the tolerance ring is mounted on a shaft prior to insertion in the bore. Such chamfers are, in themselves, already known.

A further aspect of the invention provides a blank for making tolerance ring of the first aspect. The blank comprises a strip of resilient material, in which the corrugated protrusions are formed, and having the guide portion inclined relative to the plane of the rest of the strip. The resulting blank may subsequently be curved into a substantially an annular shape corresponding to the tolerance ring of the first aspect with a gap between the ends of the strip.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described in detail, by way of example, reference to the accompanying drawings, in which:

FIG. 1 shows in cross section a bearing and a known tolerance ring, which are about to be inserted into a bore in a pivotable actuator arm of a hard disk drive;

FIG. 2 shows in cross section another known tolerance ring with outward protrusions located in a bore in an actuator arm, and a bearing ready to be inserted into the tolerance ring;

FIG. 3 shows in cross section another known tolerance ring with inward protrusions located in a bore in an actuator arm, and a bearing ready to be inserted into the tolerance ring;

FIG. 4 shows in cross section a tolerance ring of the present invention located in a bore in an actuator arm, and a bearing ready to be inserted into the tolerance ring;

FIG. 5 shows in cross section, another known tolerance ring which is fitted to the end of a shaft and is about to be inserted into a bore in a housing; and

FIG. 6 shows in cross section a tolerance ring of the present invention, which is fitted to a shaft and is about to be inserted in a bore into a housing.

DETAILED DESCRIPTION

Before describing embodiments of the invention, it will be useful to understand conventional arrangements. Thus, FIGS. 1 to 3 illustrate use of a known tolerance ring to mount a shaft in a bore.

Thus, as shown in FIG. 1, a known tolerance ring with outwardly facing protrusions 2 in the form of waves and is fitted around a bearing 3 or bearing assembly 3 (hereinafter referred to as the bearing 3). The bearing 3 and tolerance ring 1 comprise a sub-assembly, which is axially inserted into the bore 4 of a body which may be an actuator arm 5 of a hard disk drive, indicated in FIG. 1 by the arrow 6. Unformed, annular portions 7 of the tolerance ring 1, which have no radial protrusions, axially flank the outwardly facing protrusions 2. Tolerance rings with outwardly facing protrusions are generally known in the art as S.V rings (Shaft Variable), as the diameter of the tolerance ring to be fitted to the bearing may be varied by differing amounts of overlap of the ends of the strip that forms the tolerance ring. The outwardly facing protrusions 2 of the ring 1 resiliently engage the surface 8 of the bore 4.

As the sub-assembly is inserted into the bore 4, the outwardly facing protrusions 2 of the tolerance ring 1 are compressed. Unformed portions 7 of the tolerance ring 1 that axially flank the outwardly facing protrusions 2 prevent changes in the pitch of the outwardly facing protrusions 2, allowing only resilient deformation of each outwardly facing protrusions. A radial force is created between the surface 8 of the bore 4 and the bearing 3. An interference fit is thus provided by the tolerance ring 1.

During assembly, the outwardly facing protrusions 2 of the tolerance ring 1 slide relative to the surface 8 of the bore 4. Abrasion of the waves against the surface 8 can cause small fragments of the housing 5 to be removed from the surface 8 of the bore 4 (and possibly tolerance ring 1). This problem is compounded if the material of the actuator arm 5 is softer than the material of the outwardly facing protrusions 2 of the ring 1. The fragments are known in the art as particles, which can adversely affect the operation of the apparatus in which the tolerance ring 1 is fitted. In the example, particles are extremely undesirable as cleanliness is important for proper functioning of the hard disk drive.

An alternative known assembly method comprises inserting the tolerance ring 1 into the bore 4 so that the tolerance ring 1 sits concentrically in the bore 4. The bearing 3 is inserted into the bore 4 and slides into the tolerance ring 1, as shown by the arrow 9 in FIG. 2. The bearing 3 may foul on the edge 10 of the ring as the bearing 3 is axially inserted into the ring 1, causing fragments of the ring 1 and/or bearing 3 to be removed. The fragments are known in the art as particles.

Particles may also be produced when using a tolerance ring with inwardly extending protrusions, for example as shown FIG. 3. A tolerance ring 11, with inwardly facing protrusions 30, sits concentrically in the bore 4 of the actuator arm 5, with the sides of the ring engaging the sides 8 of the bore 4. This type of ring is known in the art as an H.V. tolerance ring (Housing Variable). A bearing 3 is axially inserted into the ring 11, as indicated in FIG. 3 by the arrow 12. The bearing 3 moves relative to the inwardly facing protrusions 30. As the bearing 3 is made of harder material than the actuator arm 5, particle production is more limited.

The arrangement using a tolerance ring with inwardly extending waves may not be suitable for all types of apparatus, for example where it is desirable to have a load on the bearing spread over a wider area than can be provided by the tops of the waves. It is undesirable that a requirement to minimise particle production during assembly imposes a working arrangement that is sub-optimal for the apparatus.

In a computer hard disk drive the arrangement using a tolerance ring with inwardly extending waves (H.V) is undesirable, as load must be spread over as wide an area of the bearing as possible to avoid torque ripple. Furthermore, axial alignment, required in a hard disk drive, is usually better with tolerance rings having outwardly extending waves (S.V), shown in FIGS. 1 and 2. However, manufacturers may use the H.V ring and assembly method described in Example 3, as it minimizes particle production during assembly. Particles will adversely affect the functioning of the hard disk drive.

As had previously been mentioned, a tolerance ring according to the present invention modifies known tolerance rings by providing a guide portion which is inclined relative to the axis of the tolerance ring. The first embodiment of the present invention will now be described with reference to FIG. 4. Some of the features of the tolerance ring of this embodiment are the same in the tolerance rings described previously, and the same reference numerals will be used to indicate corresponding parts.

A tolerance ring 13 being the first embodiment of the present invention allows a tolerance ring with outwardly facing protrusions 2 to be used in the apparatus but minimizes or eliminates the particle production that results from the arrangements shown in FIGS. 1, 2 and 3. The tolerance ring 13 includes a guide portion indicated generally in FIG. 4 by 14. The guide portion 14 resembles a funnel, which flares towards the entrance of the bore 4. The guide portion comprises a flared guide surface, indicated in cross section by 15 in FIG. 4, extending axially from the band 16 of the ring 13, forming an endmost annular portion, towards the entrance of the bore 4. The guide portion terminates at free end 32 defining an axial end of the tolerance ring. This free end 32 defines an opening that (i) is larger than that defined by the unformed annular portions of the band, and (ii) has a diameter at said free end that is not greater than the diameter of the protrusions. As shown, the guide portion is provided at only one end of the band.

The guide surface 15 provides an enlarged entrance to the band 16 of the ring 13 for receiving the bearing, eliminating foul on the edge of the ring as described in Example 2. Guide surface 15 is sufficiently smooth so as to guide the bearing 3 into the ring 13, even if there is some axial misalignment of the bearing 3 and the ring 13. As the outwardly facing protrusions 2 of the ring 13 do not abrade against any surface during assembly and the end of the bearing 3 does not abrade against the ring 13, particle production is avoided. As shown, the tolerance ring 13 in the form of an annular band of resilient material has an innermost surface defined by the unformed annular portions 7, and has an outermost surface defined by the outwardly facing protrusions 2 having a diameter. All of the outwardly facing protrusions 2 extend radially outward, each extending between a pair of unformed annular portions. The guide portion is contiguous with and extends axially and radially outward from an endmost unformed annular portion (the topmost unformed annular portion shown in FIG. 4), and flares outwardly from the innermost surface. As shown, the guide portion has a free end defining an opening that has a diameter not greater than the diameter of the unformed annular portions. The arrangement using an S.V. type ring is preferable for the hard disk drive.

It is also known to use a tolerance ring with inwardly facing protrusions. The ring may be fitted to the end of a shaft and inserted into a bore in the housing, as shown in FIG. 5. The tolerance ring 17, with inwardly facing protrusions 2, is fitted to a shaft 18 and the shaft 18 and ring 17 inserted axially into the bore 4 of a housing 19. The diameter of the ring 17 is similar to the diameter of the bore 4 and so the ring 17 fouls on the edge 20 of the bore 4, producing particles from the ring 17 and/or the housing 19.

This known tolerance ring can be modified by the provision of an inclined guide portion, in a manner similar to the first embodiment, but with the angle of inclination being inward rather than outward. Such an embodiment is illustrated in FIG. 6, and again reference numerals are used to indicate corresponding parts. Tolerance ring 21 of the second embodiment is fitted to a shaft 18, and axially inserted into the bore 4 of a housing 19. The ring 21 includes inward protrusions in the form of waves to the grip shaft 18.

The ring includes a tapered guide portion indicated generally in FIG. 6 by 22, extending axially from the band 23 of the ring, away from the shaft 18. The guide portion comprises a guide surface, indicated in cross section in FIG. 6 by 24 a, sloping towards the axis of the ring to narrow the opening of the band 23. The guide surface 24 a axially leads into the bore 4 during assembly. The guide surface allows correction of axial misalignment of the bearing and the bore. The guide portion prevents the ring fouling on the edge of the bore 20, reducing or even eliminating particle production.

In the embodiments described above, the bore 4 has straight side walls. It is possible for the ends of the bore to be chamfered, although the presence of the guide portion makes such chamfering of little benefit.

The tolerance ring of the invention may be used on a hard disk drive, although there are numerous other applications of the present invention, for example any apparatus that uses a tolerance ring. In the example, the hard disk drive has an actuator arm pivotable around a bearing or bearing assembly. Typically, the actuator arm has an electronic transducer at one end for reading and writing data stored on a media disk. The arm pivots around a stationary shaft, which is mounted perpendicular to the plane of rotation of the media disk and arm.

Conventionally, the pivot mechanism comprises a centre shaft and one or a plurality of rolling element bearings surrounding the shaft. Each rolling element bearing includes an inner race attached to the shaft, an outer race located concentrically around the inner race, and a plurality of rolling elements located in the annular space between the inner and outer races. Such an assembly is commonly known as a bearing cartridge assembly. An outer cylinder or sleeve may surround the outer race or races of the bearing or bearings. Although such a hard disk drive is a typical arrangement, it will be appreciated by those in the art that there are other possible arrangements to which the invention can be applied. 

What is claimed is:
 1. A tolerance ring having a first axial end, a second axial end, and a central axis, the tolerance ring comprising: an annular band; a plurality of protrusions, all of the plurality of protrusions extending from the annular band; and a guide portion extending from the annular band towards the first axial end of the tolerance ring, wherein the guide portion comprises a guide surface that is curved in an axial section.
 2. The tolerance ring according to claim 1, wherein the annular band defines an annular segment disposed between the guide portion and the plurality of protrusions.
 3. The tolerance ring according to claim 2, wherein the guide portion comprises a minimum diameter, and wherein the minimum diameter of the guide portion is coincident with the annular segment.
 4. The tolerance ring according to claim 1, wherein the annular band is adapted to contact a bearing.
 5. The tolerance ring according to claim 1, wherein the annular band is adapted to contact a shaft.
 6. The tolerance ring according to claim 1, wherein the guide portion is visible when viewed from the first axial end along the central axis of the tolerance ring.
 7. The tolerance ring according to claim 1, wherein the plurality of protrusions extend radially outwards from the annular band.
 8. The tolerance ring according to claim 1, wherein the plurality of protrusions extend radially inwards from the annular band.
 9. The tolerance ring according to claim 1, wherein the guide portion extends along a whole circumference of the annular band.
 10. An apparatus comprising: a housing having a bore therein; a shaft in the bore; and a tolerance ring positioned between the interior surface of the housing and the external surface of the shaft, the tolerance ring comprising: an annular band; a plurality of protrusions, all of the plurality of protrusions extending from the annular band; and a guide portion extending from the annular band towards the first axial end of the tolerance ring, wherein the guide portion comprises a guide surface that is curved in an axial section of the guide portion is visible when viewed from the first axial end along the central axis of the tolerance ring.
 11. The tolerance ring according to claim 10, wherein the annular band defines an annular segment disposed between the guide portion and the plurality of protrusions.
 12. The tolerance ring according to claim 11, wherein the guide portion comprises a minimum diameter, and wherein the minimum diameter of the guide portion is coincident with the annular segment.
 13. The tolerance ring according to claim 10, wherein all of a guide surface of the guide portion is visible when viewed along a central axis of the tolerance ring.
 14. The tolerance ring according to claim 10, wherein the guide portion extends along a whole circumference of the annular band.
 15. A method of assembling an apparatus, comprising: inserting a tolerance ring into a bore in a housing, the tolerance ring comprising: an annular band; a plurality of protrusions, all of the plurality of protrusions extending from the annular band; and a guide portion extending from the annular band towards the first axial end of the tolerance ring, wherein the guide portion comprises a guide surface that is curved in an axial section of the guide portion is visible when viewed from the first axial end along the central axis of the tolerance ring; and inserting an end of a shaft into the guide portion of the tolerance ring; and moving the shaft along the axis of the band into the band, so that the band engages the shaft.
 16. The method according to claim 15, wherein all of a guide surface of the guide portion is visible when viewed along a central axis of the tolerance ring.
 17. The method according to claim 15, wherein the guide portion extends along a whole circumference of the annular band.
 18. The method according to claim 15, wherein the annular band defines an annular segment disposed between the guide portion and the plurality of protrusions.
 19. The method according to claim 18, wherein the guide portion comprises a minimum diameter, and wherein the minimum diameter of the guide portion is coincident with the annular segment.
 20. The method according to claim 15, wherein the plurality of protrusions extend radially outwards from the annular band. 